UPTON, NY -- A research group led by a scientist at the U.S. Department of
Energy's Brookhaven National Laboratory has discovered a simple relationship
that mathematically links the properties of a class of high-temperature
superconductors, materials that, below a certain temperature, conduct
electricity with no resistance. This new, unexpected law applies to
superconductors with very different structures and compositions, and may provide
clues to understanding the mechanism of high-temperature superconductivity. It
is discussed in the July 29, 2004 issue of Nature.

"Because this law unifies many different materials, it may allow us to
predict the behavior of other superconductors, giving us deeper insight into how
these systems work," said Brookhaven Lab physicist Christopher Homes, who led
the research.

Homes focused on several members of a class of high-temperature
superconductors known as cuprates, which are characterized by layers of copper
oxide. He found a relationship between three of each cuprate's physical
properties: direct current, or dc, conductivity (how much direct current it
conducts); critical temperature (the temperature below which it superconducts);
and the "superfluid density" in the superconducting state. This last property
refers to how many current carriers -- electrons or "holes" (which are spaces in
the electron sea that act positively charged) -- are in the superconductor.

The new law, called a scaling relation, states that the superfluid density is
proportional to the dc conductivity multiplied by the critical temperature. When
the researchers plotted this relationship on a graph for each material and
compared the shape of these plots, they observed that the overall result is a
straight line.

"The interpretation of this result and exactly what it says about the nature
of the superconductivity in these materials is a source of ongoing debate,"
Homes said, "but it should provide insight into the origins of superconductivity
in these materials, or even give us a way to predict the behavior of other
superconductors."

The scaling relation applies regardless of each cuprate's crystal structure,
doping level (the amount of other elements, such as calcium or strontium, added
to improve their performance), and type of disorder (how the crystal lattices
are distorted by impurities). The relation also works regardless of the
direction in which the properties are measured ­ parallel to the copper
oxide planes or perpendicular to them. This is significant because, in a
cuprate, the electric current only runs parallel to the copper oxide planes.
Perpendicular to the planes, the cuprate acts much like an insulator, with very
little current flowing.

"The fact that this scaling law makes a connection between the
superconductivity in both directions, where the properties are very different,
was surprising," said Homes.

Homes and his group studied each material using a wide range of light, from
microwaves to ultraviolet rays. In some cases, they worked at the National
Synchrotron Light Source at Brookhaven, a facility that produces infrared,
ultraviolet, and x-ray light for research. They measured how much light is
reflected away from each sample, a value called absolute reflectance. From this
measurement, they calculated the properties used to determine the scaling
relation.

"The beauty of this technique is that this simple measurement allows both the
conductivity and the superfluid density to be measured without touching the
material," Homes said. "It is amazing what you can learn about a material just
by looking at how it reflects light."

Homes also applied the scaling law to two conventional superconductors, the
metals lead and niobium, using known measurements from literature. He and his
group will continue this research by studying certain other conventional
superconductors, to see if they obey the scaling law.

This research is the result of a collaboration between researchers at
Brookhaven Lab, the University of British Columbia, the Central Research
Institute of Electric Power Industry (Japan), Stanford University, the Stanford
Synchrotron Radiation Laboratory, the University of California at San Diego, and
McMaster University. The research was funded by the Office of Basic Energy
Sciences within the U.S. Department of Energy¹s Office of Science, the National
Science Foundation, the Natural Sciences and Engineering Research Council of
Canada, and the Canadian Institute for Advanced Research.

Note to local editors: Christopher Homes lives in Saint James, New
York.